Problem: On the first 5 quizzes of her geometry class, Stephanie got an average score of 82. What does she need on the next quiz to have an overall average of 85?
Solution: Let her score on the next quiz be $x$ The sum of all of her scores is then $5 \cdot 82 + x$ The same sum must also be equal to $6 \cdot 85$ Solve: $x = 6 \cdot 85 - 5 \cdot 82 = 100$.